1. Field of the Invention
The present invention relates to a laser, and more specifically, to a laser having a resonator structure which efficiently generates only a fundamental transverse mode.
2. Description of the Related Art
Lasers have been widely used in scanners or the like in various fields, such as for reading information from a recording medium or for writing information therein, and lasers having higher powers are demanded.
As a method of obtaining a high-power laser, an oscillation mode volume increase in an amplification medium or in an active region is used. When this method is practically and simply carried out, it is effective to increase a transverse-direction volume of the amplification medium or a gain thereof.
However, when a power increase is attempted by using this method, transverse modes in higher orders generally oscillate, and oscillation of only a single transverse mode cannot be realized effectively. Therefore, it is impossible to focus an oscillating laser light to its diffraction limit, and a power density of the focused laser light is not proportional to an increase in an oscillation power. This problem is especially conspicuous in high-gain semiconductor lasers. As a high-power semiconductor laser, a broad-area semiconductor laser having a wide active region has been known. This device does not have a resonator structure having a mode selectivity such that only a single transverse mode can oscillate efficiently. Therefore, a multitude of high-order transverse modes oscillate at the same time and an oscillating laser light cannot be focused to its diffraction limit. As a result, a focused spot having a high power density cannot be obtained.
As a method to solve these problems, a variety of resonator systems to improve the mode selectivity have been proposed. Hereinafter, these resonator systems will be explained briefly.
(1) Spatial Filter Method
A technique wherein higher-order mode losses are increased relatively and a fundamental transverse mode in a lowest order is caused to oscillate efficiently by installing a spatial filter comprising a lens and a simple aperture has been known (see xe2x80x9cHigh-Radiance Room-Temperature GaAs Laser With Controlled Radiation in a Single Transverse Mode (E. M. Phillip-Rutz, IEEE J. Quantum Electron QE-8,632 (1972)xe2x80x9d, xe2x80x9cHigh-power, diffraction-limited, narrow-band, external-cavity diode laser (W. F. Sharfin, J. Seppala, A. Mooradian, B. A. Soltz, R. G. Waters, B. J. Vollmer, and K. J. Bystrom Appl. Phys. Lett. 54,1731 (1989)xe2x80x9d, Japanese Patent Application Publication No. 4(1992)-504930 or the like).
However, such a laser can only relatively increase the higher-order mode losses, since the laser uses a simple single aperture. Therefore, spatial frequencies in the higher-order modes cannot be eliminated completely out of the resonator. As a result, the higher-order modes can be oscillation modes of the resonator when a gain is increased, despite the fact that the optical losses are increased. For this reason, an amplification medium having a high enough gain, such as a semiconductor laser, cannot suppress oscillation of the higher-order modes, and a plurality of higher-order modes oscillate together with the fundamental transverse mode.
(2) Mode Selecting Mirror Method
Another technique wherein only a desired spatial mode is caused to oscillate by using a resonator mirror having mode selectivity has also been known. As lasers using this technique, diffractive mode-selecting mirror methods 1 and 2 or the like have been known (see xe2x80x9cHigh modal discrimination in a Nd:YAG laser resonator with internal phase gratings (J. R. Leger, D. Chen, and K. Dai, Opt. Lett. 19,1976 (1994)xe2x80x9d, xe2x80x9cLarge-area, single-transverse-mode semiconductor laser with diffraction limited super-Gaussian output (G. Mowry and J. R. Leger, Appl. Phys. Lett. 66,1614 (1995)xe2x80x9d). However, they also have the same problem as the spatial filter method lasers do.
(3) Beam Addition Method
As a laser which solves the problems that the spatial filter method and the mode selecting mirror method have and causes a high-power fundamental transverse mode to oscillate, the present applicant, for example, has proposed a coherent beam addition laser which adds a plurality of laser beams onto a common axis and causes the added beam to oscillate as a single laser beam (see Japanese Unexamined Patent Publication No. 8(1996)-76054 (Japanese Patent Application No. 5(1993)-336318), xe2x80x9cCoherent beam addition of GaAlAs lasers by binary phase gratings (J. R. Leger, G. J. Swanson, and W. B. Veldkamp, Appl. Phys. Lett. 48,888 (1986)xe2x80x9d, xe2x80x9cCoherent addition of GaAlAs lasers using microlenses and diffractive coupling (J. R. Leger, M. L. Scott, and W. B. Veldkamp, Appl. Phys. Lett. 52,1771 (1988)xe2x80x9d, U.S. Pat. Nos. 4,649,351, and 4,813,762, and the like).
According to this technique, a special optical device called an array illuminator for adding a plurality of laser beams onto a common axis is used.
As an array illuminator, an optimally-designed phase grating, a polarizing device, a Fourier transform lens array or the like can be used. According to the coherent beam addition laser, the quality of a beam emitted from each laser medium is reflected in the quality of the added laser beam. Therefore, in order to obtain a fundamental transverse-mode beam, each laser medium needs to have a structure to emit only the fundamental transverse mode.
However, it is generally difficult and costly to produce a laser medium wherein a plurality of laser media emitting the fundamental transverse mode alone are laid out in an array.
According to the conventional techniques described in the above, it is difficult to obtain a preferable beam-quality laser causing only the fundamental transverse mode to oscillate efficiently, since a laser using a laser medium having a wide active region or a large mode volume generates a multitude of higher-order transverse modes at the same time. Therefore, it is difficult for a high oscillation-power laser to focus an oscillating laser beam to its diffraction limit and to obtain a high power density.
Based on consideration of the above problems, an object of the present invention is to provide a high-power, preferable beam-quality laser by causing only a fundamental transverse mode to oscillate efficiently, even when a laser medium having a wide active region or a large mode volume is used therein.
A laser of the present invention places a predetermined array illuminator optical system within its resonator structure, separates a component of a higher-order mode from a fundamental transverse mode component by diffracting the higher-order mode in an angle larger than a predetermined diffraction angle, and eliminates the higher-order mode component out of the resonator. In this manner, only the fundamental mode component can oscillate, generating a laser beam.
In other words, the laser of the present invention comprises:
a laser resonator optical system; and
an array illuminator optical system placed within the resonator optical system, the array illuminator optical system placing a plurality of complex amplitude distributions similar to that of an incident laser beam at even spacing and in a uniform phase in a plane perpendicular to an optical axis of the laser resonator.
The term xe2x80x9csimilar toxe2x80x9d herein referred to means a state wherein a pattern the same as a fundamental complex amplitude distribution occurs in a space smaller than that of the fundamental distribution, and this meaning is the same as generally used in geometry or the like.
As the array illuminator optical system, an array illuminator optical system comprising:
a Fourier plane array illuminator using a first lens array or a first phase grating;
a Fourier transform lens for carrying out a Fourier transform on a complex amplitude distribution of a laser beam having passed through the Fourier plane array illuminator; and
a Fourier plane array illuminator using a second lens array or a second phase grating which corrects a phase of and collimates each laser spot in a laser spot array formed at even spacing on a Fourier transform plane of the Fourier transform lens can be applied.
When the array illuminator optical system comprises the Fourier plane array illuminator using the first lens array, the Fourier transform lens, and the Fourier plane array illuminator using the second lens array, it is preferable for the array illuminator optical system to be set such that
fxc3x97p2=f2xc3x97p1xc3x97A2xe2x80x83xe2x80x83(1)
fxc3x97p1=f1xc3x97p2xc3x97A1xe2x80x83xe2x80x83(2)
(p1xc3x97p2)/(xcexxc3x97f)=Ixe2x80x83xe2x80x83(3)
(where I is an integer other than 0)
are satisfied. Here, f1, p1, and A1 mean a focal distance, a pitch, the number of generated complex amplitude distributions of the Fourier plane array illuminator using the first lens array respectively, and f is a focal distance of the Fourier transform lens. Likewise, f2, p2, and A2 are a focal distance, a pitch, the number of generated complex amplitude distributions of the second lens array respectively, and xcex means a wavelength of the laser beam.
As the laser resonator optical system, a ring resonator can be used as well. Furthermore, the laser beam is not limited to a specific one, and all kinds of laser beams can be adopted.
According to the laser of the present invention, the array illuminator optical system within the resonator optical system places a laser beam incident thereon as a plurality of complex amplitude distributions similar to that of the laser beam in an equal interval and in a uniform phase in a plane perpendicular to the optical axis of the laser resonator. When the incident laser beam has a complex amplitude distribution in a fundamental transverse mode, the complex amplitude distributions placed at the even spacing and the uniform phase are added onto the optical axis to generate a wave front having the uniform phase, which propagates along the optical axis. Meanwhile, when the incident laser beam has a higher-order transverse mode, the complex amplitude distributions placed at the even spacing and the uniform phase are added to generate a wave front having a spatial frequency higher than the frequency determined by the even pitch. Therefore, the wave is diffracted and deviated from the optical axis in an angle larger than the angle determined by the pitch. Therefore, the fundamental transverse mode component having the constant phase can propagate along the optical axis without a loss in light quantity, and a laser beam is generated by the resonator optical system. At this time, the higher-order transverse mode component is not a target of laser-beam oscillation caused by the resonator optical system.
The laser of the present invention can generate a laser beam including only a fundamental transverse mode component in the uniform phase, even when a laser medium having a wide active region or a large mode volume is used therein. Therefore, a high-power, high-quality laser beam can be obtained.